As a technique to automatically distinguish whether a passenger seated in a seat is an adult or a child and to perform a feedback to an actuation control section of an air bag, for example, in order to automatically switch actuation control of the airbag, there is known a seat which has a mechanism for automatically detecting weight of a person. Usually, since a urethane foam is used as a cushioning member, a displacement amount of a coil spring supporting that urethane foam, the coil spring being disposed in a vertical direction in relation to a vehicle body floor, is measured to detect weight by the amount of displacement. On the other hand, the present applicant has disclosed a technique as Patent Document 1 in which a cushioning member provided in a seat cushion section is elastically supported by a torsion bar disposed in the seat cushion section and the torsion bar has a load fluctuation detecting device incorporated therein. More specifically, there has been disclosed a technique in which a pickup coil is wound on the above-described torsion bar and a reference bar (magnetic bar member) which is not distorted by a load fluctuation is provided, a pickup coil is wound also on the reference bar, and a load fluctuation is detected by calculating a difference of induced voltages obtained from two coils.
Patent Document 1: Japanese Patent Application Laid-open No. 2005-345257